1. Field of the Invention
The present invention relates to a battery charger and a method of detecting a fully charged condition of a secondary battery such as a nickel-cadmium battery and a nickel-hydrogen battery.
2. Description of the Related Art
Various methods have been proposed in the art to detect the fully charged condition of a secondary battery. One is to sample battery voltage at every predetermined timing. When the peak voltage appearing at the charge termination period is detected, it is determined that the battery has reached a fully charged condition. This method will hereinafter be referred to as xe2x80x9cpeak voltage detection methodxe2x80x9d. Another method is to detect battery temperature at every predetermined timing and compute a rate of temperature rise, that is, a temperature rise gradient. When the temperature rise gradient has exceeded a predetermined value, the battery is determined to be fully charged. This method will hereinafter referred to as xe2x80x9cdT/dt detection methodxe2x80x9d.
The peak voltage detection method is not suitable for the batteries which exhibit battery charge characteristic with no clear peak voltage. Such batteries include a nickel-hydrogen battery.
The dT/dt detection method, on the other hand, may fail to detect the fully charged condition of the battery. In the dT/dt detection method, the temperature rise gradient is compared with a fixed critical value. As such, detection of the fully charged condition of the battery is made based, among other things, only on the temperature rise gradient. Other factors, such as the kind of the battery to be charged, the condition of the battery, battery temperature at the time when charging starts, charge current, or ambient temperature, are not considered for determining the fully charged condition. Those unconsidered factors may increase the battery temperature rise gradient more than the fixed critical value despite the fact that the battery has not yet reached the fully charged condition. In such a case, charging is stopped before the battery is fully charged, so the battery is undercharged. On the other hand, the battery temperature rise gradient may not increase more than the fixed critical value despite the fact that the battery has reached the fully charged condition. In this case, the battery is overcharged because charging will not stop even if the battery is fully charged. Overcharging the battery may cause electrolyte to leak out from the battery attendant to gas generation occurring at the charge termination period. This shortens a cycle lifetime of the battery.
If with the dT/dt detection method, the critical value used for evaluating the temperature rise gradient is varied depending on the kind of the battery to be charged, the condition of the battery, battery temperature at the time when charging starts, charge current, or ambient temperature, the battery charger employing the dT/dt detection method and the control of the battery charger will become complicated.
Accordingly, it is an object of the present invention to provide a battery charger and a method of accurately detecting a fully charged condition of a secondary battery regardless of the kind of the battery to be charged, the condition of the battery, battery temperature at the time when charging starts, charge current, or ambient temperature.
To achieve the above and other object, there is provided, according to one aspect of the present invention, a controlling method of a battery charger, including the steps of:
a) starting charging a battery;
b) sampling a battery temperature at every predetermined timing;
a) computing a change in battery temperature rise gradient each time the battery temperature is sampled; and
d) determining that the battery has reached a fully charged condition based on a transition changing from increment to decrement of the change in battery temperature rise gradient.
Step d) may include the steps of:
d1) obtaining a maximum value of the change in battery temperature rise gradient at every sampling of the battery temperature; and
d2) determining that the battery has reached the fully charged condition when an updated value of the change in battery temperature rise gradient falls a predetermined value from the maximum value.
Step d) may include the steps of:
d3) detecting that the change in battery temperature rise gradient exceeds a first predetermined value:
d4) after step d3), detecting that the change in battery temperature rise gradient falls below a second predetermined value; and
d5) after step d4), determining that the battery has reached the fully charged condition.
According to another aspect of the invention, there is provided a battery charger which includes: a battery temperature sensing device for sensing a battery temperature and outputting a battery temperature signal indicative of the battery temperature; sampling means for sampling the battery temperature signal at every predetermined timing; computing means for computing a change in battery temperature rise gradient and outputting an updated value of the change in battery temperature rise gradient each time the battery temperature is sampled; determining means for determining that the battery has reached a fully charged condition based on a transition changing from increment to decrement of the change in battery temperature rise gradient.
In one embodiment, the determining means designates a maximum value of the change in battery temperature rise gradient at every sampling of the battery temperature, and determines that the battery has reached the fully charged condition when the updated value of the change in battery temperature rise gradient falls a predetermined value from the maximum value.
In another embodiment, the determining means detects that the change in battery temperature rise gradient exceeds a first predetermined value and thereafter detects that the change in battery temperature rise gradient falls below a second predetermined value, whereupon the determining means determines that the battery has reached the fully charged condition.